I have had great help here with the AAC community. Thank you all for your support and ideas helping my projects become a reality.

I am looking to create a audio mixer of sorts. The idea is ... there will be 3 audio inputs, out to one audio output. The inputs will be a car stereo of 20 watts RMS and (2) soundboards with 7 watts RMS. These inputs will merge to ONE audio output. This Audio output is for a LED Spectrum graph that responds to the audio being sent to it.

These 3 inputs will be tapped (Parallel) from a speaker that will also be playing the audio. It would be nice to have a trimmer pot for volume inputs. There will NOT be a selector switch to choose which input will go to the output. ALL 3 inputs will delivery to the same audio output, whenever an audio is played from ANY 1 of the 3 inputs.

Is this possible to do without Audio interference and popping noise spiking through the LED Spectrum graph when a audio becomes present to the output?

I have done something like this before. I placed (2) separate 7 watt peak audio outputs to one 20 watt RMS amplifier input. I had to use Electro caps to make it work without a popping sound during playback. It worked fine.

But what I propose now, seems to be a little more complex. And I am at a lost on where to start.

The car stereo use an IC chip for its amplified system. The negative output comes from IC, not directly from ground. .... I actually haven't seen a common ground car speaker system since the 1980's.

I have contacted the manufacture of the soundboard to confirm if the audio negative output is a ground. I did use my Fluke meter on resistance and measured nothing between ground and the speaker output, while the soundboard was off.

Or, use only 1 of the two bridge outputs through a coupling capacitor. As long as it is not class-D relying on the speaker for filtering (like the TI "filterless" class D parts), the two bridge outputs are analog signals 180 degrees out of phase.